US 7333205 B2
A system and method for generating and using broadband surface plasmons in a metal film for characterization of analyte on or near the metal film. The surface plasmons interact with the analyte and generate leakage radiation which has spectral features which can be used to inspect, identify and characterize the analyte. The broadband plasmon excitation enables high-bandwidth photonic applications.
1. A plasmon excitation and sensor system for detecting surface material on a substrate, comprising:
a source of light for exciting asymmetrical surface plasmons in the surface material;
an optical coupling element receiving the light from said source and then further receiving and outputting leakage radiation emitted by the surface plasmons interacting with the surface material; and
a radiation sensor for receiving and analyzing the leakage radiation to determine characteristics of the surface material.
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13. A method of sensing characteristics of an analyte on or near an asymmetrical plasmonic surface, comprising the steps of:
applying light selected from the group of a white-light continuum and a set of discrete wavelengths to a substrate with analyte disposed at least one of adsorbed on a metal surface and near a metal surface;
forming asymmetrical surface plasmons in the substrate;
sensing leakage radiation emitted by decaying of the surface plasmons; and
analyzing the leakage radiation to characterize the analyte.
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18. A plasmon excitation and sensor system for detecting surface features disposed on a substrate, comprising:
a source of focused and polarized light;
at least one of an immersion objective lens and another focusing element in optical contact with the surface feature and for receiving at least one of a white-light continuum and a discrete set of wavelength and coupling the white-light continuum and the discrete set of wavelengths to an asymmetrical surface plasmon in the substrate and the surface features;
the at least one of immersion objective lens and another focusing element further operating to receive leakage radiation emitted by surface plasmons created in the substrate by the light and outputting the leakage radiation; and
at least one of a CCD camera system and another optical detector for receiving and analyzing the leakage radiation to characterize the surface features.
This application claims priority to U.S. Provisional Patent Application No. 60/666,901 filed on Mar. 31, 2005, and is incorporated herein by reference.
The United States Government has certain rights in this invention pursuant to Contract No. W-31-109-ENG-38 between the U.S. Department of Energy and The University of Chicago operating Argonne National Laboratories.
This invention relates to a method for optically exciting and detecting surface plasmons in thin metal films, schematically illustrated in
A plasmon is the quantization of plasma oscillations, which are density waves of the charge carriers in a conducting medium such as a metal, semiconductor, or plasma. Surface plasmons exist in various geometries, such as nanoparticles or two dimensional films. Thin film plasmons propagate in the micron range depending on the wavelength and type of material. Such plasmons are non-radiative in air and are sensitive to dielectric environment. Recent technological advances that allow metals to be structured and characterized on the nanometer scale triggered new interest in the application of surface plasmons (SPs). The control of SP properties is of interest to a wide spectrum of scientists, ranging from physicists, chemists and materials scientists to biologists. For instance, SPs are being explored for their potential in small-scale optical circuitry, high-resolution optical microscopy and bio-detection.
Surface plasmons are known solutions of Maxwell's equations applied along an interface between a medium with a negative permittivity, i.e. a metal, and a dielectric. These solutions are traveling waves that are generally bound to the interface and are exponentially decaying in both media. The optical excitation of surface plasmons on flat metal interfaces is challenged by the phase matching condition between the plasmons and the exciting radiation. The surface plasmon dispersion ω(k) is located outside the light cone ω=ck and hence no SPS can be excited with freely propagating radiation. The excitation of surface plasmons can only occur if the photon momentum—or the wave vector—can be artificially increased. Various experimental techniques have been developed to accomplish this task, such as (i) increasing the index of refraction of the incident medium (total internal reflection (TIR) conditions) or (ii) engineering the surface of the film (grating coupler). While these approaches provide very efficient coupling between the incident photons and the SP waves, the interaction area is usually comparable or greater than the SP propagation distances.
It was recognized very early that in an asymmetric structure, i.e. a thin metal film (permittivity εm) surrounded by two dielectric media (permittivities ε1, and ε2, with ε1>ε2), has four modes that are solutions of the dispersion relations. Two of these solutions exist at each of the interfaces εm/εi, i=1, 2) and are characterized by their fields decaying exponentially into the media. The two other modes are radiative leaky waves originating from the finite thickness of the film. As a non-radiative mode travels along an interface, the wave amplitude decays exponentially in the metal and is coupled into leakage radiation (“LR”) by the opposite interface. The far-field observation of this leakage radiation (LR) gives a direct measurement of the non-radiative surface plasmon propagation at the opposite interface. The intensity of the radiation, at a given lateral position in the film, is proportional to that of the SP—at the same position.
Surface plasmons are thus well-known phenomena and commercial surface plasmon-based sensors are currently used in biological research and in industrial applications. Use of the surface plasmons allows manipulation of light in devices smaller than the wavelength and can be extremely localized. They also exhibit ultrafast dynamics for use in rapidly changing circumstances or for rapid data output. For example, the detection principle of a commercially available plasmon sensor relies on the surface plasmon resonance resulting from energy and momentum being transformed from incident photons into surface plasmons. This process is sensitive to the refractive index of the medium on the opposite side of the film from the reflected light. Heretofore, the light source used for optically exciting surface plasmons was a monochromatic laser directed at an angle through a prism to a metal (gold or silver) coated surface. The sensor operated by determining the variation in the angle of incidence for maximum plasmon absorption. The presence of an adsorbate material on the surface of the metal was detected by measuring the change in the angle of incidence of the monochromatic beam. Alternatively, the angle of incidence was fixed and the wavelength varied to extract the same information. These methods are, however, very time consuming and technically more difficult if one wishes to extract spectral information, e.g. multiple wavelengths, on the adsorbates material.
Further, in the vast majority of other surface plasmon studies, they are optically excited in the so-called Kretschmann attenuated total internal reflection (“ATR”) configuration, where the momentum mismatch between free-propagating photons and SPs is taken from a material with a refractive index larger than air, e.g. a glass substrate. In the case of an ATR geometry and for smooth metal films, the leakage radiation (“LR”) interferes destructively with the incoming excitation light at the reflection spot and cannot be detected if the excitation area is larger or comparable to the lateral decay length of the surface plasmon. However, if surface plasmons are locally excited by electrons or near-field techniques, LR can be observed. This is, however, technologically ambitious and difficult because near-field optics is based on scanning probe microscopy.
The invention is directed to a new system and method for the excitation and detection of a large spectrum of surface plasmons. With the increasing trend towards the miniaturization of photonic circuits, the confined nature of surface plasmons and their long propagation length make them suitable for integration in metallic planar circuitry designs. Rudimentary surface plasmon (“SP”) optical manipulations in structured thin films include propagation, interference, scattering, waveguiding, splitting and mirror-like reflection. Due to the evanescent nature of the SP field traveling at the metal/air interface, near-field and fluorescence techniques were applied to image the surface plasmon intensity distributions and have been essential in the characterization of SP devices.
The preferred technique utilizes an incident white-light continuum beam as a excitation source and an index-matched immersion objective lens having a wide aperture in contact with the substrate being probed. The objective can be part of a conventional inverted optical microscope focused on the metal/glass interface. Importantly, the focusing of the white light continuum through the microscope objective produces a wide range of incident angles of excitation, so as to simultaneously launch a continuum of plasmons. This method and system enables an unusually large bandwidth or frequency of plasmons to be excited rather than the very narrow bandwidth of previous methods. Detection of the surface plasmon continuum is achieved by monitoring the leakage radiation including a continuum of wavelengths, including the visible and infra-red spectral region which has originated from the propagation of the surface plasmons. The radiation is recorded by a conventional CCD camera placed in an image plane. The instantaneous detection of the wide radiation bandwidth permits a new form of spectroscopy of adsorbates on the surface of the metal film.
A plasmon sensor system 10 constructed in accordance with a predefined form of the invention as illustrated in
As shown in
A spatial variation of the spectral components of the surface plasmon 11 produces a rainbow-like jet in the collected images for the resonance conditions of
The leakage radiation 22 (See
The oil immersion objective 20 we used has a most preferred numerical aperture (N.A.) of 1.4, meaning that the angular spread ranges between 0° to 68°. The SP excitation angles for wavelengths throughout the visible are confined within a few degrees around 45°. Therefore, if the full N. A. of the objective 20 is used, only a small fraction of the light 13 will be converted into the surface plasmons 11; and the overwhelming remaining part will be reflected or transmitted through the silver film 14. Instead of completely filling the back-aperture of the objective 20, a small beam of the collimated white-light beam 13 was adjusted within the back-aperture of the objective 20 as depicted in
The incident white-light beam 13 continuum was produced by the output of a Coherent MIRA regeneratively amplified Ti:Sapphire laser system (not shown). The beam 13 is created through well-known methods, in particular by focusing the 800 nm pulses into a small piece of sapphire (50 fs/pulse at 250 kHz). The white light beam 13 produced in this manner is generally easier to manipulate, collimate, and focus than other typical white light sources. The polarization of the beam 13 was controlled by a conventional multi-wavelength waveplate (not shown). The asymmetric plasmonic films were produced by thermally evaporating about 45±5 nm thick silver films on cleaned ones of the glass cover slips 16.
The resulting plasmon sensor system 10 is a highly sensitive device which can analyze and detect extremely small quantities of adsorbates on a metallic conducting material. Various features of surface plasmon sensors can be exploited to determine the presence and amount of adsorbates and even near surface constituents which are different than the matrix of the material being studied. An example of plasmonics is shown in
Further work is illustrated in
One embodiment of the invention of
It should be understood that various changes and modifications referred to in the embodiment described herein would be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present invention. For instances, the femtosecond laser system used to produce the white-light continuum can be replaced by a simple gas bulb (halogen, etc.), or light emitting diode (LED), or emissive element (tungsten or carbon for example), etc. Similarly, the broad spectrum of wave vectors produced by the objective lens can also be produced by a defect on the film (engineered or natural) that is sub-wavelength in dimensions, or by the proximity of a near-field probe (with or without aperture). Similarly, a solid immersion lens or other high numerical aperture optic can readily replace the oil immersion objective used here. Similarly, the research grade CCD can be replaced by simpler devices, such as a digital camera, diode, or integrated hand-held or on-chip spectrograph. The inverted microsocope is used only for versatility and exploring a range of initial optical configurations during research. Now optimized, it can be eliminated in a commercial system. Changes to the detection of the broadband leakage radiation can also be readily envisioned by those skilled in the art, e.g. by avoiding leakage radiation collection by the objective lens.